1. Field of the Invention
The present invention relates to a projection type image display apparatus using a light modulation image display device including a liquid crystal display device as a light valve.
2. Description of the Prior Art
For image display on a wide screen, hitherto, a method of forming an optical image corresponding to a video signal changes the optical characteristics of on a relatively small light valve, illuminates this optical image with illuminating light and magnifies and projects this image on a screen using a projection lens is widely known. In this sort of projection type image display apparatus, the resolution of the projected image is determined by the resolution of the light valve, and when the light source is intensified, the light output becomes larger, and therefore by using a light valve of high resolution, a projection type image display apparatus of large light output and high resolution may be realized even if the display area is small. Recently, a method of using a liquid crystal display device as the light valve is noticed. For example, Morozumi et al. proposed, in "LCD Full-Color Video Projector," SID 86 Digest, p. 375, a method of obtaining a full-color projected image by using three liquid crystal display devices.
The structure of a conventional example of projection type image display device using such liquid crystal display device as the light valve is described below. The light released from the lamp is converted into a nearly parallel light by a focusing optical display device, and passes through the liquid crystal display device and enters the projection lens. In the liquid crystal display device, an optical image corresponding to a video signal obtained from an image display circuit is formed as change in the transmissivity, and this optical image is projected on the screen by the projection lens. As the lamp, a halogen lamp, a xenon lamp, or a metal halide lamp may be used.
The image display circuit comprises a video signal demodulation circuit, a common voltage generation circuit for generating a voltage to be applied to the counter-electrode of the liquid crystal display device (hereinafter called the common voltage), and a video signal output circuit for inverting the polarity of the video signal demodulated in the video signal demodulation circuit at field intervals with respect to the common voltage in order to drive the liquid crystal display device with an alternating current.
In such a conventional projection type image display apparatus, however, owing to the peripheral dimming characteristics of the projection lens, assuming that the image angle of the projection lens to the screen to be .omega., the brightness of the peripheral part as compared with the central part on the screen of the projected image is lowered in proportion to cos.sup.4 .omega..
Besides, in order to correct the luminance gradient due to the difference between the effective voltage of the signal applied to the upper portion of the screen of the liquid crystal display device and the effective voltage of the signal applied to the lower portion, a method of applying a correction voltage to the common electrode was proposed (see Japanese Patent Publication Sho. 62-217224).
When the liquid crystal display appearance is used in the projection type image display apparatus, however, the peripheral dimming characteristics of the projection lens are not fully corrected by the correction of the luminance gradient in the vertical direction of the screen alone.